RFR: 8355400: Better git detection in update_copyright_year.sh

[Erik Joelsson]

On Wed, 23 Apr 2025 13:19:19 GMT, Manuel Hässig <duke at openjdk.org> wrote:

> The script `make/scripts/update_copyright.sh` relies on the path to the script for git detection, which is fragile or does not work at all or git worktrees. This PR uses git itself to detect whether the script is running in a git repository. This has the added benefit that the script can be executed from anywhere in the repository.

Marked as reviewed by erikj (Reviewer).

-------------

PR Review: https://git.openjdk.org/jdk/pull/24828#pullrequestreview-2787576772